In a magnetic recording disk drive, data is stored in a thin magnetic layer on the disk. Data is written to and read from the disk by a read/write head on a head carrier or slider that is maintained in close proximity to the rotating disk.
The magnetic recording disk typically comprises a substrate, such as an aluminum-magnesium (AlMg) alloy disk blank with a nickel-phosphorous (NiP) surface coating or a chemically-strengthened glass disk blank, a cobalt-based alloy magnetic layer, and a protective overcoat of amorphous carbon, hydrogenated-carbon and/or nitrogenated-carbon for corrosion resistance and wear resistance from the slider. A liquid lubricant is also maintained on the carbon overcoat to prevent damage to the head and the disk during starting and stopping of the disk and inadvertent contact of the slider with the disk. The lubricant is typically applied by dipping the disk into a solution of the lubricant in a fluorinated solvent and then evaporating the solvent. During normal operation of the disk drive, wear is prevented by lubricant reflow onto regions of the disk from which lubricant has been removed by sporadic contacts with the slider.
The conventional disk lubricants comprise mixtures of long chain polymers characterized by a wide distribution of molecular weights and include perfluoropolyethers, functionalized perfluoropolyethers, perfluoropolyalkylethers (PFPE), and functionalized PFPE. The PFPE lubricants have polar hydroxyl end groups that physisorb and chemisorb on the carbon overcoat. Within the context of magnetic recording, chemisorbed lubricant is the lubricant that remains on the carbon overcoat after rinsing with solvent.
One problem with the lubricants is that they tend to deplete due to air shear forces, mechanical shear forces from the slider, and spin-off from centrifugal forces during operation of the disk drive. To address this problem certain PFPE lubricants are available that have a relatively high molecular weight (MW), e.g., greater than 2500, to increase the viscosity and thus decrease the tendency to spin off the disk. An example of this type of lubricant is a Z-Dol manufactured by Solvay Solexis Sp.A., Italy. Z-Dol-type lubricants have dual functionality with 1° hydroxyl end groups and are random copolymers of perfluoromethylene, ethylene, propylene, and butylene oxide. Since the lubricant chain contains very little perfluoropylene and butylene oxide, the general structural formula for a Z-Dol is given by:R—CF2—O—(CF2O)m—(CF2—CF2—O)n—CF2—Rwhere m and n are integers and R=—CH2OH.
The performance of Z-Dol-lubricated disks can be improved if the molecular weight of the Z-Dol applied to the disks can be controlled. Conventional well-known extraction processes for isolating specific molecular weight fractions of the Z-Dol to be applied to the disks are distillation and supercritical fluid extraction. Liquid/liquid extraction is a separation process that takes advantage of the relative solubilities of solutes in immiscible solvents. The solute dissolves more readily and becomes more concentrated in the solvent in which it has a higher solubility. A partial separation occurs when a number of solutes have different relative solubilities in the two solvents used. In U.S. Pat. No. 5,292,585, liquid/liquid extraction has been proposed for a Z-Dol derivative, AM-2001, for removing impurities and low molecular weight AM-2001 using an ester and an alcohol as the solvents, with the high molecular weight AM-2001 being generally insoluble in the alcohol.
However, with the increase in disk drive operating speeds to 10,000 RPM and higher, lubricants such as Z-Dol and AM-2001 cannot be sufficiently prevented from the effects of high mechanical shear, high air shear and centrifugal forces. Thus, newer PFPE lubricants have been proposed, such as Z-Tetraol, also manufactured by Solvay Solexis, in which the adhesion force of the lubricant to the carbon overcoat is made stronger by increasing the polarity of the functional end group. Z-Tetraol has mostly di-hydroxyl end groups consisting of 1° and 2° hydroxyl groups. The general structural formula for Z-Tetraol is the same as for Z-Dol but the end group is given by:R=—CH2OCH2CH(OH)CH2OH
Z-Tetraol as purchased from the manufacturer has a wide distribution of molecular weights. Z-Tetraol is derived from Z-Dol by addition of glycidol onto the Z-Dol end-groups. Typically only about 85% of the as-purchased Z-Tetraol has the desired di-hydroxyl end groups, with the remainder having the Z-Dol end group. As a result, disks lubricated with as-purchased Z-Tetraol will have lubricant having a mix of Z-Tetraol and Z-Dol end groups.
Thus isolation of the Z-Tetraol end groups and isolation of specific molecular weight fractions of Z-Tetraol from the as-purchased Z-Tetraol are desired. Higher molecular weight Z-Tetraol may be desirable, while Z-Tetraol with a higher percentage of di-hydroxyl end groups is desirable to improve the consistency and tenacity of the bonding of the lubricant to the carbon overcoat across the disk. In addition, low molecular weight Z-Tetraol has a higher volatility and may be desirable for disk manufacturing where the lubricant is applied to the disks in the vapor phase, as an alternative to the conventional dipping method.